Much effort is currently being directed towards novel and renewable sources of energy which do not rely on fossil fuels.
One such area of research is the process known as pressure retarded osmosis (PRO). In this process, a semipermeable membrane is used to separate a less concentrated solution from a more concentrated solution. The membrane causes solvent to pass from the less concentrated solution (with low osmotic pressure) to the more concentrated solution (with high osmotic pressure) by osmosis, and this leads to an increase in pressure on the side of the membrane to which the solvent diffuses. This pressure can be harnessed to generate electricity. A small number of PRO plants are in operation around the world, and these generally use differences in salinity as the driver for osmosis, typically using fresh water from a river or lake as the feed stream for the less concentrated solution, and sea water for the more concentrated solution. Helfer et al, J. Membrane Sci. 453 (2014) 337-358 is a review article describing PRO. Typically, PRO schemes to date have used seawater and river water mixing, and in pilot-scale plants the process has been found to be uneconomic due to low power densities achieved. It has been suggested that a power density of around 5 W/m2 membrane represents a level of power generation above which PRO may become economically viable. Outside of laboratories it has not generally been possible to achieve this level of power density using existing membrane technology in river/seawater mixing schemes.
A number of attempts have been made to harness the energy found in underground formations in processes involving osmosis. WO 2013/164541 describes a method for generating power by direct osmosis, in which the more concentrated solution is “production water”, while the less concentrated solution is fresh water or sea water. Production water is water obtained after separation from a hydrocarbon stream during hydrocarbon production. WO 2013/164541 also mentions that a brine stream obtained from an underground formation can be used as the more concentrated solution.
However, most attempts to generate power by osmosis and also to harness the energy present in geothermal streams use a completely different approach. This is described in a number of documents which envisage using the heat obtainable from geothermal sources as a driver for closed-loop osmosis systems. US 2010/0024423 explains the difference between an “open loop” PRO system in which the feeds are typically fresh water and sea water and the spent solutions are discharged back into the environment, and “closed loop” system in which a single solution is separated, for example by evaporation, into a more-concentrated and a less-concentrated solution. Such separation requires energy, which may be supplied by low-grade heat sources such as industrial waste heat, or renewable heat sources such as geothermal heat sources. The particular invention of US 2010/0024423 is a closed loop osmotic system in which the draw solution is ammonia and carbon dioxide. Other documents describing a closed loop system in which a heat transfer step is used to separate a solution into more-concentrated and less-concentrated solutions, the heat being supplied from a geothermal source, include US 2014/0026567 and Lin et al, Environ. Sci. Technol. 2014, 48, 5306-53113.
No known process, however, harvests the maximum available amount of energy latent in the warm saline streams present in underground geothermal formations. We have now found a process capable of increasing the efficiency of energy extraction from warm saline streams present in underground geothermal formations.